1. Field of the Invention
The present invention concerns an organic material with an extremely narrow electron spin resonance (ESR) line and its application to the making of gaussmeters or magnetometers.
2. Description of the Prior Art
To enable the making, notably, of magnetic anomaly probes, working in the earth's magnetic field, various research centers have undertaken studies of certain organic materials with extremely narrow ESR lines. The desired ESR line widths are smaller than 20 mG.
Radical-cation salts derived from certain condensed aromatic hydrocarbons such as fluoranthene (FA) and perylene (PY) have remarkable electron spin resonance (ESR) properties. For (FA).sub.2 PF.sub.6 for example, a remarkably narrow ESR signal at ambient temperature has been observed.
This material has an absorption derivative peak-peak line width of only 10 milligauss. The concerned absorption signal is therefore practically 100 times narrower than the ESR signal of the diphenylpicrylhydracyl radical (DPPH) commonly used as an ESR field standard. However, while the absence of heteroatoms in these materials may be favorable to the narrowness of the ESR line widths, it is, on the other hand, a factor that causes a certain degree of thermal instability. Thus, for example, salts prepared from naphthalene are stable only below -40.degree. C., while the corresponding ESR line width is the narrowest (4 milligauss) for this entire series of materials.
The remarkable narrowness of these electron spin resonance lines should enable the use of these materials to make very highly sensitive magnetometers provided that their thermal stability in time, which is at very insufficient at present, is notably improved.
With respect to substituted perylenes, only a small number of alkylperylenes has been synthesized. These are the three monomethyl regiosoisomers, 1-(n butyl)perylene, 1- and 2- ethylperylene, 2.8-, 3.9- and 2.11-dimethylperylenes. The alkylperylenes with higher substitution levels are rare, and only 1, 2, 7, 8-tetrahydrocyclopenta [cd:1m] perylene has been synthesized with low efficiency (N. TANAKA, T. KASAI, Bulletin of the Chemical Society of Japan, 1981, Vol. 54, p. 3026).
The article "Radical-cation salts of substituted perylenes" by R. LAPOUYADE, J. P. MORAND, D. CHASSEAU, C. HAUW and P. DELHAES in the Journal De Physique (Conference C3, supplement to No. 6, Vol. 44, June 1983 pages C3-1235 to C3-1238) concerns a radical-cation salt of 1, 2, 7, 8-tetrahydrocyclopenta {cd:1m}.
This substitute perylene may be more simply called CPP. The stoichiometry of the salt in question is CPP).sub.2 . +PF.sub.6 -, CH.sub.2 Cl.sub.2.
According to the authors of this article, this salt has an absorption derivative peak-peak line width of the order of 80 milligauss.
In order to obtain a material which has good thermal stability at ambient temperature while, at the same time, having an ESR line width smaller than (20) mG and high stability in air, the applicant has done a great deal of work on different salts of organic materials and, especially, on substituted perylenes. The applicant has taken up and has extended studres made on, among other materials, 1, 2, 7, 8- tetrahydrocyclopenta {cd:1m} perylene or CPP. The conclusions of this research have proved to be astonishing. For, the following CPP salt, (CPP;.sub.2.sup.. +PF.sub.6 -, CH.sub.2 Cl.sub.2, which had been stated, in the above-quoted article "Radical-cation Salts of Substituted Perylenes" by R. LAPOUYADE, J. P. MORAND, D. CHASSEAU, C. HAUW and P. DELHAES in JOURNAL DE PHYSIQUE (Conference C3, supplement to No. 6, Vol. 44, June 1983, pages C3-1235 to C-1238), as having an ESR line width of 80 mG, has in fact proved to have ESR line width of 16 mG.
Owing to the declared line width (80 mG) which was defined only to characterize the material, its use to make magnetic field probes for gaussmeters or magnetometers could not be contemplated.
The applicant has pursued research on the material in question and has found that an appropriate heat treatment has, contrary to all expectations, highly beneficial consequences for the ESR line width and, hence, for the possibilities of use of this material.